I have a diamond solitaire platinum ring & I clean it with hot water with a dash of ammonia in & a tooth brush (recommended by my jeweller).
:yeahthat: That's how I do mine too (not platinum though, 18ct white gold). Our jeweller has made all of our rings and told us to clean them all this way. I then rinse them in hot water and dry them with a hairdryer. They come up super shiny and the diamonds are very sparkly

For what it's worth, he told us to only have it cleaned and polished once a year because over time it causes too much wear on the jewellery. I usually get mine done around our anniversary.